Low-emissivity coatings for glass and other substrates are well known in the present art. Typically, they include one or more infrared-reflective layers each positioned between two or more transparent dielectric layers. The infrared-reflective layers reduce the transmission of radiant heat through the coating (e.g., by reflecting infrared radiation). These infrared-reflective layers typically comprise conductive metals, such as silver, gold, or copper. The transparent dielectric layers are used primarily to reduce visible reflectance and to control other coating properties, such as color. Commonly used transparent dielectrics include oxides of zinc, tin, and titanium, as well as nitrides, such as silicon nitride.
In most cases, each infrared-reflective layer in a low-emissivity coating comprises silver. Silver is the most commonly used infrared-reflective material because it provides high electrical conductivity (and hence low-emissivity), high visible transmission, and neutral color. A drawback of using silver for each infrared-reflective layer is that silver lacks mechanical and chemical durability. Silver layers are very soft and thus limit the mechanical durability of silver-based coatings. Silver layers are also particularly vulnerable to becoming corroded. Thus, great care must be exercised to prevent silver-based coatings from being damaged. For example, consider manufacturing periods (e.g., prior to and/or during assembly of coated substrates into IG units). During these periods, coated substrates are frequently subjected to relatively harsh conditions. For example, the conditions associated with handling, shipping, and washing can cause silver-based coatings to become scratched or otherwise abraded. During manufacturing periods, coated substrates are also commonly exposed to air, moisture, and other chemicals, all of which can cause silver to become corroded. Thus, when pure silver layers are used in a low-emissivity coating, the overall durability of the coating tends to be less than ideal.
Attempts have been made to enhance the durability of infrared-reflective layers. For example, some have replaced the inner and outer silver layers with layers of a more durable reflective metal. Others have replaced the inner and outer silver layers with layers of a silver alloy comprising a small amount of a more durable reflective metal. For example, alloys of silver and palladium have reportedly been found to create infrared-reflective layers with greater durability than pure silver. These alternatives, however, have largely been rejected in the marketplace, as they are predominately viewed as yielding unacceptably high emissivity. Therefore, pure silver is typically used for each infrared-reflective layer in a low-emissivity coating, notwithstanding its mechanical and chemical vulnerability.
The properties of an infrared-reflective silver layer depend upon the surface over which it is deposited. For example, a silver layer can be grown to have particularly low emissivity by depositing the silver layer directly over a film of pure zinc oxide. Thus, it is a widespread practice in the art to position each infrared-reflective silver layer in a low-emissivity coating directly over a pure zinc oxide layer.
While zinc oxide is beneficial for growing a high quality silver film, it has several drawbacks. One known drawback is that, because zinc oxide is a highly crystalline film, it is not particularly dense. Thus, pure zinc oxide layers tend to be less than ideal for preventing air, moisture, sodium ions, and other materials from migrating through the zinc oxide layers and potentially reaching and reacting with the silver layers. Further, when zinc oxide is deposited by sputtering, it tends to exhibit pinholes more frequently than would be ideal. Great care is taken to avoid pinholes, as they can also give air, moisture, and other chemicals access to the silver layers. Another drawback of zinc oxide is that thick zinc oxide layers tend to exhibit more stress than is preferred. This can result in less than optimal adhesion, hence creating the potential for delamination. Notwithstanding these drawbacks, it is conventional in the art to provide pure zinc oxide directly beneath each silver layer in a low-emissivity coating.
It would be desirable to provide a low-emissivity coating that achieves better durability than conventional low-emissivity coatings wherein each infrared-reflective layer is pure silver. It would be particularly desirable to provide a coating that achieves this result without an undue increase in emissivity.